例如第一个字符是r 114，那么： select * from student s where id=1 and 1=~113+ascii(substring(current_user,1,1)) 而： select * from student s where id=1 and 1=~114+ascii(substring(current_user,1,1)) 不报错，并且之后>114的也不会报错，因此第一位就是114
r
select * from student s where id=1 and 1=~113+ascii(substring(current_user,1,1))
select * from student s where id=1 and 1=~114+ascii(substring(current_user,1,1))